A variety of applications use power supplies to provide electrical power to a load. A power supply may receive electrical power from a line source, such as a wall outlet, and convert the power for use in a particular application. A load may respond to the delivery of power in a linear manner, such as a purely resistive load, or the response may be nonlinear. An induction motor that is used to drive typical exercise equipment is one example of a nonlinear load.
In one aspect, a power supply may convert an input line current from alternating-current (AC) to direct current (DC), and vice versa. A power supply may also be configured to boost the voltage supplied to a load prior to delivering power to the load. A load will often require power to be supplied at a constant voltage.
FIG. 1 illustrates a prior art circuit 10 that attempts to provide power to a load at an approximate constant voltage. The circuit 10 includes an AC power source 12 connected to a full-wave bridge rectifier circuit 14. The rectifier circuit 14 is comprised of a set of diodes 16, 18, 20, and 22. When the voltage signal from the power source 12 is positive with respect to point AC1, current flows through diodes 16 and 18, charging the capacitor 24 as shown. When the voltage of the power source 12 is negative, current flows through diodes 20 and 22. The function of the rectifier circuit 14, in this case, is to convert the negative part of the sinusoidal input signal to a positive one, as shown by line 28 in FIG. 2A. The original sinusoidal input signal is shown by line 30. A capacitor 24 is connected to the output of the rectifier circuit 14 and, once charged, tries to maintain a constant voltage Vout at the output during the operation of the circuit 10. The power supply circuit 10 is designed to deliver power to a load 26, here depicted as a resistor Rload.
FIG. 2B illustrates a problem that occurs with the prior art circuit of FIG. 1. In FIG. 2B, the sinusoidal voltage signal of the AC power source 12 is depicted by line 30. Though not illustrated in FIG. 2B, the input signal 30, once rectified as shown in FIG. 2A, appears as two positive half-waves. When the capacitor 24 is connected to the output of the bridge rectifier 14, the capacitor acts as a voltage reservoir and transforms the input voltage 30 into the output voltage Vout depicted by line 34. During portions of each half-cycle of the input voltage 30, the output voltage 34 (i.e., the voltage on the capacitor 24 in FIG. 1) is greater than the input voltage 30. When the input voltage 30 exceeds the capacitor voltage 24, current flows from the power source 12 to the capacitor 24 and the load 26. The input current, shown as line 32, is drawn from the power source 12 with a large peak to average ratio. The sudden increase and decrease in current as shown by line 32 results in large harmonic content in the current waveform. The large peak to average ratio of the current also produces losses in the circuit, such as heating. The result is that the power drawn from the power source 12 may be much more than the power that can actually be used by the load 26.
The prior art has attempted to overcome this inefficiency in power transfer by use of power factor correction (PFC) circuits. The definition of power factor is the cosine of the angle between voltage and current waveforms and generally refers to the ratio of actual power drawn from a power source to the usable power in the load (i.e., the product of the voltage and current in the load). A circuit as shown in FIG. 1 may have a low power factor ranging from 0.5 to 0.7, where with power factor correction circuitry, the power factor may be increased and begin to approach a maximum power factor of 1.0.
Usable power in a load derives from components of the current and voltage waveforms that are in phase with each other. If an input current waveform is distorted from the input voltage waveform, as shown in FIG. 2B, the current waveform will have components at frequencies other than the frequency of the voltage. These components do not contribute to the usable power received by the load. They do, however, contribute to the average current drawn from the power source 12. PFC circuits reduce the harmonic content of the current waveform and minimize the phase angle between the input current and voltage so that the usable power received by the load is closer to the actual power drawn from the power source.
PFC circuits are widely known in the art and are generally considered a requirement for most off-line power supplies. PFC circuits may be comprised of active and/or passive components. Passive PFC circuits rely on a combination of inductors and capacitors to shape the current waveform. While a passive circuit is generally less complicated and less expensive to build than an active circuit, it is difficult to optimize a passive circuit for universal line operation.
One example of a prior art circuit with active PFC components is illustrated in FIG. 3. The circuit 40 is connected to an AC power source 42 via a rectifier circuit 44. For ease of description, the rectifier circuit 44 may be similar to the rectifier circuit 14 shown in FIG. 1. The circuit 40 uses a switch 46 (for example, a transistor) that controls the flow of current, and hence the input current waveform, drawn from the power source 42. When the switch 46 is closed (i.e., conducting), input current flows through an inductor 48 and the switch 46 to ground. When the switch 46 is “open” (i.e., non-conducting), current flows from the inductor 48 through diode 52 to the capacitor 50 and the load 76. Electrical current at the output of the circuit 40 is supplied by the inductor 48, the capacitor 50, and the power source 42. The inductor 48 and capacitor 50 act as energy storage components that help boost the output voltage Vout and maintain it approximately constant.
The operation of the switch 46 is controlled by the circuitry depicted in the lower portion of FIG. 3. The switch 46 is generally opened and closed at a frequency much higher than the line frequency of the power source 42. As will be discussed briefly below, the output voltage Vout is typically monitored and compared to a predetermined desired output so that the switch 46 can modulate the input current and maintain the desired output voltage. The input current waveform is also modulated by the switch 46 so that it more closely follows the input voltage waveform. Conventional techniques known in the art for modulating switch operation in a PFC circuit include pulse-width modulation (as shown in FIG. 3) and frequency modulation.
More specifically, the circuit 40 shown in FIG. 3 includes a multiplier 54 with three inputs. The first input (on line 56) is a measure of the input current sensed by sense resistor 58. The input on line 60 is a measure of the input voltage sensed by the sense resistor 58, which is filtered by a low pass filter 61 and squared by squaring circuit 62. Lastly, the input on line 64 is a measure of the output voltage sensed by sense resistors 66 and 67, and compared to a reference voltage Vref by a comparator 68.
The output of the multiplier is amplified and filtered by circuitry 70 and delivered to a pulse width modulator (PWM) 72. The output of the pulse width modulator 72 directs the gate driver logic 74 to produce a signal that opens and closes the switch 46. Because the construction and use of conventional PFC circuits, such as the one shown in FIG. 3, is widely known, persons having ordinary skill in the art will recognize the operation of the circuit 40 without further detail being provided herein. The output voltage and current from the circuit 40 is delivered to a load, such as load 76, which may be linear or nonlinear in nature.
While FIG. 3 depicts a circuit with active components for power factor correction, the circuit is also exemplary of deficiencies that arise in the prior art. Prior art PFC circuits are implemented in hardware, using multiple interconnected components typically formed of multiple integrated circuits, e.g., as shown in FIG. 3. Each of the integrated circuits has its own limitations and operating characteristics. Because of this, prior art PFC circuits are generally designed for a narrow range of input and output power and cannot handle power conversion over a wide range. Expanding the range of power that conventional PFC circuits can handle and still maintaining a constant power output is very expensive and difficult to implement.
The prior art is also limited in that conventional PFC circuits are implemented separately from other power circuits. For instance, it is often desired to take a boosted DC power produced by a PFC circuit and connect it to an inverter that produces an AC signal for driving an induction motor. Having a PFC circuit and an inverter implemented separately on different circuit boards, as done in the prior art, increases the complexity of the overall motor control circuit, is more susceptible to electrical noise, and is more difficult to cool, usually resulting in increased cost and size of the circuitry.
The invention described herein addresses these deficiencies and other shortcomings in the prior art.